Population Dynamics

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Population Dynamics
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Populations are dynamic

• Size
• Density
• Age distribution
• Dispersion (spatial pattern)

These changes (population dynamics) occur in
response to environmental stress
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Dispersion patterns within populations
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Why clumping?

• Resources vary from place to place
• Moving in groups allow better chance finding
  resources
• Living in groups protects some animals from
  predators
• Hunting in packs give some predators an advantage
  of finding and capturing prey
• Some species form temporary groups for mating and
  caring for young

Raft of sea otters
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Four variables which govern population size

• Births
• Immigration
• Death
• Emigration

Population Change (Births Immigration)
(Deaths Emigration)
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Age Structure

• How fast a population grows or declines depends
  on its age structure.
• Prereproductive age not mature enough to
  reproduce.
• Reproductive age those capable of reproduction.
• Postreproductive age those too old to reproduce.

Baby hippos stay with their mother for 4 years
and reach their reproductive age at about 5 6
years
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What would you say about this population of fish
if they are capable of reproducing between the
ages of 10 and 20 years? What would you say
about this population if they did not reach
reproductive age until the age of 26 years?
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Factors affecting population size

• Intrinsic rate of increase
• Environmental Resistance
• Carrying Capacity
• Minimum Viable Population

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Limits on population growth

• No population can increase its size indefinitely.
• The intrinsic rate of increase (r) is the rate at
  which a population would grow if it had unlimited
  resources.
• Carrying capacity (K) the maximum population of
  a given species that a particular habitat can
  sustain indefinitely without degrading the
  habitat.

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Factors affecting carrying capacity

• Competition for resources
• Immigration and emigration of other species
• Natural and human-caused catastrophes
• Seasonal fluctuations

Lake Lanier, GA in Spring 2008
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What happens when a population exceeds its
carrying capacity

• Dieback (crash)
• Easter Island
• Cause damage reduce areas carrying capacity
• Overgrazing by cattle
• Reduces grass cover in some areas
• Allows nonnative species to inhabit the area
  (opportunists)

Sagebrush invaded after overgrazing occurred by
cattle (the cattle do not eat the sagebrush)
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Minimum viable population
The smallest possible size at which a population
can exist without extinction from natural
disasters, genetic changes, human influence, or
environmental factors
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If a population falls below minimum viable
population (MVP) needed to support a breeding
population

• Individuals may not be able to locate mates
• Genetically related individuals may interbreed
• Genetic diversity may be too low

California condor
The intrinsic rate of increase falls and
extinction is likely
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Genetic diversity can affect the size of a
population

• Founder effect
• When only a few individuals in a population
  colonize a new habitat that is geographically
  isolated
• Demographic bottleneck
• When only a few individuals in a population
  survive a catastrophe (i.e. fire or hurricane)
  and this lack of genetic diversity may limit
  ability to rebuild population
• Genetic drift
• Random changes in gene frequencies where some
  individuals breed more than others and their
  genes may eventually dominate the gene pool
• Inbreeding
• Individuals in a small population mate with one
  another

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Under some circumstances population density
affects population size

• Density-independent population controls
• Their effect is not dependent on the density of
  the pop.
• Floods, hurricanes, severe drought, seasonable
  weather, fire, habitat destruction, and pesticide
  spraying
• Density-dependent population controls
• Competition for resources, predation, parasitism,
  and disease

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Density-dependent population controls

• Higher density may help find mates, but can lead
  to increased competition for mates, food, living
  space, water, sunlight, and other resources

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Density-dependent population controls

• Can help shield members from predators, but it
  can also make large groups such as schools of
  fish vulnerable to human harvesting methods

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Density-dependent population controls

• Close contact among individuals in a dense
  population can increase the transmission of
  parasites and infectious disease

April 2010 Connecticut deer population is 64
deer/ square mile the deer populations density
is contributing to the spread of Lyme disease to
humans (they would like to get the deer
population down to 12 deer/ square mile
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Several different types of population change
occur in nature

• Population sizes may stay the same, increase,
  decrease, vary in regular cycles, or change
  erratically.
• Stable fluctuates slightly above and below
  carrying capacity.
• Irruptive populations explode and then crash to
  a more stable level.
• Cyclic populations fluctuate and regular cyclic
  or boom-and-bust cycles.
• Irregular erratic changes possibly due to chaos
  or drastic change.

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Cyclic fluctuations (boom-and-bust cycles)
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Top-down population control

• Predators (high on trophic pyramid) regulate prey
  population size
• Example of top-down effects
• Increase in number of predators
• Reduces size of herbivore population
• Increases size of plant population
• Reduces nutrients in soil

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Bottom-up population control

• Each trophic level derives its energy from the
  level below it
• Productivity of the species on the lower layer
  sets the bounds on the population sizes of the
  species on the higher layer
• Example of bottom-up effects
• Increase nutrients in soil
• More nutrients increases size of plant population
• More plants increases herbivore population
• More herbivores increases size of predator
  population

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Lynx-Hare Population Cycle
Is it top-down or bottom-up?
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How does productivity influence top-down or
bottom-up population control?

• With high primary productivity
• Higher primary productivity results in herbivore
  populations large enough to support prey
  populations
• Predators suppress the herbivore populations
  top-down regulation
• With low primary productivity
• When primary productivity is low, there is not
  enough production of biomass to sustain large
  herbivore populations bottom-up regulation

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Reproductive Patterns
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Survivorship Curves
K-selected species
r-selected species
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